Thermal fire detection has been achieved by a number of means over the years. Detectors are divided into two basic types: spot type and linear. The spot type detectors are individual devices which incorporate a small switching mechanism. These devices are mounted typically on the ceiling and wired to a control panel. They are most effective for the detection of fires in rooms, etc. However, there are many applications where the spot type detectors are not cost effective. Often the ability to detect a fire needs to be applied in spaces where the spot detector cannot be located or spread out over a specific area such as a cable tray, or engine housing. For this type of protection, linear detectors have been used.
Linear thermal detection has been available in three (3) technologies. The first is the thermistor cable. This is a coaxial cable which employs a negative temperature coefficient thermistor as the insulator between the outer and inner conductor. When the cable is heated the resistance of the thermistor decreases, and, therefore, the current between the core and jacket increases. This technology is expensive and requires an extremely sensitive electronic amplifier to generate useable data.
The second technology is the fusible insulation cable. This consists of two metallic conductors which are coated with thermoplastic insulation of specific melting point. The two insulated conductors are formed together into a twisted pair cable. When the temperature of the cable reaches the fusion temperature of the insulation, the latter melts and the residual spring tension of the wire forces the two conductors together resultino in a short circuit. This technology has one disadvantage in that once the insulation has melted that section of wire must be removed and replaced with a new section. Other problems include conductor fatigue, corrosion, R.F.I. susceptibility and dielectric breakdown.
The third technology employs pneumatic tube detection. This consists of a tube of thermoplastic material which contains a gas at elevated pressure. When the tubing reaches its fusion temperature due to fire or overheat condition the tube melts releasing the gas contained within. This causes a pressure reduction in the control unit precipitating the alarm and extinguishing agent discharge. These systems are also expensive to install and suffer from severe leakage problems.
Thus, there is an obvious need for a light weight, reliable, inexpensive linear thermal detection technique. None of the currently available techniques satisfy the needs of the industrial fire protection market. Such detection technique would ideally use a detection mode which could be placed along cable trays, engine housings, transformer shells, roller bearing mounts and the like to detect overheat conditions and fires. The sensing in the detection device must be flexible, light and reasonably strong. The detection device must also be resistant to vibration, attack by water and other solvents, and to electro-magnetic interference.
The present invention utilizes fiber optics to address certain of the problems experienced in this field in the past.
Particularly, this invention seeks to utilize optical fibers, plastic or fused silica core, in a novel fashion to detect changing environmental conditions as, for example, fire or an overheat condition. Plastic fibers are suitable for situations where relatively shortruns of fiber are acceptable--because of fairly high transmission losses (on the order of 300 dB per kilometer, e.g. robotic or engine applications. For larger more extensive hazard areas, e.g. conveyor systems, refinery columns, tanks, etc., the more efficient, silica core fibers are utilized (transmission loss of 10 dB per kilometer). However, in order to provide an adequate signal to noise ratio on longer length fibers, the modal dispersion ratio (MDR) of the fiber must be adjusted.
The MDR of a fiber refers to the ratio of the unreflected radiant power to the total radiant power introduced into the fiber--reflected and unreflected. "Reflected" radiant power refers to the internal reflection of the introduced radiant power which occurs at the core-cladding interface of the fiber.
For purposes of this invention, so-called multimode fibers, having a step-index construction (discrete interface between core and clad) are the better choice. These exhibit lower transmission losses and have much lower MDR's than the single mode, type fibers which have been developed and are designed to maximize the MDR.
Further, even though the multimode fiber is preferred over the single mode fiber, an improvement in the signal to noise ratio and thus the sensitivity of the fiber optic detector, is still required for many practical applications.
It is, therefore, a primary object of this invention to provide a condition responsive detector system which employs fiber optics.
It is also an object of this invention to provide a fire and thermal detector system which employs fiber optics.
It is yet another object of this invention to provide a fire and thermal detection system which utilizes multimode fibers having lower modal dispersion ratios relying on the melt-through of the cladding and the consequent loss of reflected radiant energy to detect fires and overheated conditions.
It is still another object of this invention to provide a means for increasing the reflections off the core-cladding interface to thus lower the MDR of the fiber optics as to further enhance the sensitivity of the system.